1. Field of the Invention
The present invention relates to a process for depositing a thin film on a thin film of oxide superconductor, more particularly, to a process for improving crystallinity of a surface of the thin film of oxide superconductor so that another thin film grows epitaxially on the surface of oxide superconductor.
2. Description of the Related Art
A superconducting compound oxide of (La,Sr).sub.2 CuO.sub.4 which exhibits the superconductivity at 30 K. was discovered in 1986 by Bednorz and Muller (Z. Phys. B64, 1986 p 189). Then, another superconducting material of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. having the critical temperature of about 90 K. was discovered in 1987 by C. W. Chu et al. (Physical Review letters, Vol. 58, No. 9, p 908) and Maeda et al discovered so-called bismuth type superconducting material of Bi--Sr--Ca--Cu--O (Japanese Journal of Applied Physics, Vol. 27, No. 2, p 1209 to 1210). The other high-temperature compound oxide systems are also reported.
These superconducting compound oxides are expected to be utilized in electronics devices such as Josephson element or superconducting transistors due to their high critical temperatures (Tc). In order to realize such electronics devices, it is indispensable to deposit a plurality of thin films of superconducting oxide on a substrate successively. For example, in the case of so-called tunnel type Josephson Junction having a layered structure of superconductor/non-superconductor/superconductor, at least three layers of a thin film of the first superconductor, a thin film of the non-superconducting and a thin film of the second superconductor must be deposited successively on a substrate.
In the tunnel type Josephson Junction, a thickness of the non-superconductor layer is determined by or depends on the coherent length of superconductor used. However, in the case of the thin film layer of oxide superconductor, the thickness of the non-superconductor layer must be as thin as several nanometers (nm) because the coherent length of oxide superconducting is very short, so that it is requested to prepare a very thin film of non-superconductor.
Still more, in order to realize a Josephson Junction of high performance, such very thin film of non-superconductor must have good crystallinity of high quality. In fact, from the view point of performance of electronics devices, it is preferable that all thin film layers of superconductor/non-superconductor/superconductor have good crystallinity of high quality and are preferably made of single crystals. In other words, if one of the thin film layers is of polycrystal or amorphous, the resulting Josephson Junction including such polycrystal or amorphous thin film layer(s) shows poor performance and, in the worst case, does not work.
The thin films having good crystallinity of high quality are also required in a process for fabricating the superconducting transistors in which superconductor is combined with semiconductor.
However, it has been difficult to prepare a layered structure of superconductor/non-superconductor/superconductor in which all layers possess high crystallinity without spoiling the superconducting property in the superconductor layers by known process because of the following reason:
When a first thin film of oxide superconductor deposited is exposed to air, both of superconductivity and crystallinity are deteriorated or lost to a depth of about 1 nm at a surface of the first thin film. In the prior arts, the deposited first thin film of oxide superconductor is inevitably exposed to air, because deposition of the first thin film of oxide superconductor (for example, prepared by sputtering) and deposition of another thin film to be layered thereon (for example, prepared by vacuum evaporation) are carried out in different chambers, so that the first thin film of oxide superconductor is necessarily exposed to air during transportation from one chamber to another chamber.
In order to solve this problem, in the prior arts, the first thin film of oxide superconductor is heat-treated at about 700.degree. C. in ultra-high vacuum of about 1.times.10.sup.-9 Torr before another thin film is deposited thereon. It is true that this heat-treatment improves crystallinity of the surface of the first thin film of oxide superconductor and hence the upper thin film to be deposited thereon can be grown epitaxially.
However, such heat-treatment carried out in such ultra-high vacuum has such a drawback that oxygen atom which is a constituent element of the first thin film layer of oxide superconductor is lost, resulting in that the superconducting property of the first thin film is deteriorated and, in the worst case, disappears.
The superconducting property may be maintained if the heat-treatment is carried out in oxygen atmosphere. In this case, however, crystallinity of the surface of the first thin film of oxide superconductor can not be improved or becomes worse.
Therefore, an object of the present invention is to solve the problems and to provide a process to improve crystallinity of the surface of the first thin film of oxide superconductor on which another thin film is to be deposited without spoiling superconducting property of the first thin film.